Revert notifications_from table

I didn't think this change through and it's going to be too difficult
to implement right now. It can wait until we do the notifications
revamp. For now reverting back to the old structure, with the
`sender_id` column renamed to `from_user_id`.

src/Notification/NotificationRepository.php

@@ -32,7 +32,7 @@ class NotificationRepository
         )
             ->where('user_id', $user->id)
             ->whereIn('type', $user->getAlertableNotificationTypes())
-            ->whereNull('deleted_at')
+            ->where('is_deleted', false)
             ->groupBy('type', 'subject_id')
             ->orderByRaw('MAX(created_at) DESC')
             ->skip($offset)